1. Field of the Invention
The present invention relates to an alternator mounted on a vehicle such as a passenger car or a truck.
2. Description of Related Art
In recent years, there has been an increasing demand for vehicle alternators driven by an engine mounted on a vehicle, which generate less magnetic noise when generating power, in order to contribute to upgrading the vehicle. However, on the other hand, there has been an increasing demand for vehicle alternators having high power generation capacity because of an increase of electrical load due to an increased number of safety control devices and the like mounted on the vehicle. Accordingly, it has been required to provide small-sized, low-noise, and high-output vehicle alternators at a lower cost.
To address such requirements, various methods for reducing magnetic noise of a vehicle alternator have been proposed. For example, Japanese Patent Application Laid-open No. 4-26345 describes a structure in which two sets of three-phase windings are disposed spaced by an electrical angle of 30 degrees. This structure aims to reduce magnetic noise due to the sixth harmonic of a magnetomotive force of the stator. On the other hand, Japanese Patent Application Laid-open No. 2001-238374 describes a structure in which two sets of three-phase windings are provided such that their phase coils are located at alternate intervals of an electrical angle of 32.5 degrees and 27.5 degrees. This structure aims to remove the drawback of the structure disclosed in the above Japanese Patent Application Laid-open No. 4-26345 that although the sixth harmonic of the magnetomotive force can be reduced, the twelfth harmonic of the magnetomotive force increases.
One of the structures described in the former patent document and the structure described in the latter patent document can be selected in accordance with use conditions of an alternator. That is, when the sixth harmonic of the magnetomotive force is the only issue, it is appropriate to use the structure described in the former patent document. On the other hand, when the twelfth harmonic of the magnetomotive force is significantly large, it is appropriate to use the structure described in the latter patent document. The use conditions which make such a difference include a resonance frequency of an alternator mounting structure, and a rotation speed of an alternator.
It may occur to take a scheme of determining the electrical angle between two adjacent phase coils in between the electrical angles shown in these patent documents depending on the rigidities of an engine and an alternator mounting structure, etc. However, since it is desirable to standardize the shape and size of a stator core from a production view point, it is difficult to implement such a scheme. In addition, since the frequency at which magnetic noise can be reduced (or cannot be reduced) varies depending on a rotation speed of an engine, it is impossible to use an alternator driven by the engine always in an optimum state.